1. Field of the Invention
The present invention relates to gamma ray imaging, and in particular, to nuclear medical imaging using Compton scattering of gamma rays simultaneously emitted by a radio-nuclide.
2. Background of the Invention
Nuclear medical imaging is an important research and clinical tool. The two common techniques used are Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). These are extensively used to investigate the function of various organs and to determine the location and morphology of malignant tissue. Both SPECT and PET techniques use radio-nuclides which are administered to a patient. In SPECT a variety of radio-pharmaceuticals are used to study various tissue functions such as heart and kidney function and to locate cancer tissue. An exemplary application of PET imaging includes the use of positron emitting isotopes such as 15O and 18F to investigate brain function.
In SPECT applications, a radio-pharmaceutical is administered to a patient. Nuclei of the radio-pharmaceutical decay with the emission of gamma rays. The location of the gamma ray emission is imaged using collimated, position-sensitive gamma ray detectors. Each detected gamma ray is assumed to have arrived at the detector through the collimator. A single SPECT detector can provide a two-dimensional image of the distribution of the radio-nuclides. By moving the detector to obtain many views of a region-of-interest from different directions or using several detectors to simultaneously view the region-of-interest from different directions, coupled with the use of computer algorithms, it is possible to reconstruct a three-dimensional structure of the radioactivity.
SPECT is most frequently used with relatively low-energy gamma rays because it is difficult to fabricate collimators that are effective for gamma rays above several hundred keV. A very widely-used radio-nuclide for SPECT is 99mTc which emits a 140 keV gamma ray with a half-life of approximately six hours. Other commonly used radio-nuclides used with SPECT include 67Ga (93 keV), 111In (172 and 245 keV), 131I (364 keV), and 201TI (70-80 keV X-rays).
A major limitation with SPECT is the small field-of-view provided by the collimators and the associated very low fraction of the gamma rays that pass through the collimator to the detector. Typically only 0.001%-0.01% of the gamma rays pass through the collimator to the detector. This results in a trade-off between sensitivity and resolution of SPECT imaging. Coarse collimators transmit a higher percentage of the gamma rays but with a poor image resolution. For example, an average image resolution for many clinical applications is around one cm. Conversely, fine collimators provide improved imaging resolution, e.g. around three to five mm, but with lower sensitivity and/or higher radiation doses to the patient. Because of the low detection efficiency, rather large doses of tens of milli-curies must be administered to the patient to achieve adequate images.
In PET, a positron-emitting radio-nuclide is administered to a patient. Commonly used PET radio-nuclides include 11C(xcfx841/2=10 m), 15O(xcfx841/2=2 m), and 18F(xcfx841/2=110 m). Positrons interact with electrons in the surrounding tissue of the patient and emit two 511 keV gamma rays in opposite directions, at an angle of almost exactly 180 degrees. These gamma rays interact in position-sensitive gamma ray detectors, and restrict the origin of the gamma rays to the line joining the two detection positions. Accumulation of a large number of such events along many differing lines of position enables the reconstruction of a three-dimensional image of the radioactivity. The position resolution in PET is typically three to five mm, and is limited by the position resolution in the detectors and the range of the positrons in the body of the patient.
An advantage of PET over SPECT is that higher efficiency is achieved since collimators are not required. However, a disadvantage of PET is a higher cost of PET systems relative to SPECT systems.
Both SPECT and PET have the disadvantage that for each nuclear decay event detected (i.e., a single gamma ray in SPECT and coincident 511 keV gamma rays in PET), the location of the radio-nuclide is only determined to a line within the region-of-interest. As a result, a large number of events (e.g., thousands to millions) must be recorded and processed by computers to generate a three-dimensional image of the region-of-interest. Due to the requirement of recording a large number of events, coupled with the relatively low detection efficiencies for each radio-nuclide decay, the overall capabilities of both SPECT and PET are limited.
One method to overcome the very low gamma ray throughput of the collimators and the associated increased radiation dose required in SPECT includes electronic collimation. In electronic collimation, Compton scattering is used to reconstruct three-dimensional images from the processing of Compton direction cones of individual gamma rays detected.
An example of implementing electronic collimation for imaging includes a Compton imager that detects multiple Compton scattering interactions in arrays of 1-2 mm thick silicon strip detectors. Specifically, the energies and positions derived from the multiple Compton scattering interactions in the detectors are analyzed for consistency with Compton kinematics to select the correct interaction sequence and thereby determine the most probable incoming direction cone for the gamma ray. Because of the low efficiency of silicon arrays, an alternate electronic collimation imaging concept includes a calorimeter where scattered gamma rays leaving the silicon array are totally absorbed.
Previous Compton imaging concepts are based on operating in a single photon mode. That is, the three-dimensional image reconstruction is achieved by back-projecting Compton direction cones into the presumed source volume and using computer algorithms to reconstruct the morphology of the radioactivity. In this approach, each decay leads to a large conical-shaped annular volume within which the decay occurred.
An alternative Compton imaging concept uses detectors formed of a plurality of thin position-sensitive silicon detector layers. A Compton scattered electron is tracked through two or more layers of the detectors. With this information, the direction of the incident gamma ray can be reduced from a cone to a segment of a cone, with the advantage of improved image reconstruction.
One disadvantage of this alternative Compton imaging system includes a compromise in energy determination of the scattered electron at a first interaction site, and therefore, an associated degradation in the imaging resolution, due to compromised energy resolution in the multiple, thin detector layers. Another significant limitation of this imaging system is the difficulty of tracking electrons with energies below about one MeV due to multiple scattering events. As a result, the most commonly used nuclear medical radio-nuclides cannot be employed by this alternative concept.
A further Compton imager concept uses noble gas detectors. This imager assumes a first interaction that is a Compton scatter event followed by a photoelectric, full energy absorption, at the second interaction site. Advantages of this approach include the use of large volume, position-sensitive detectors, and the moderately good energy resolution. A significant disadvantage is the rather low efficiencies achievable with gas detectors. Another disadvantage is a poorer imaging resolution resulting from the moderate energy resolution of gas detectors.
A limitation relating to most presently available Compton imager concepts is that these devices require that the incident gamma ray energy is totally absorbed. This is necessary to properly determine the direction cone from the Compton scatter formula at the first interaction site. Requiring that the full energy of the incident gamma rays be absorbed will usually result in reduced efficiencies, especially for gamma rays above several hundred keV.
A further limitation of Compton imaging concepts is a xe2x80x9cDoppler broadening effectxe2x80x9d associated with Compton scattering itself. The Doppler broadening effect results from including the pre-interaction momentum of the electron at the first interaction site, thereby compromising the angular (image) resolution. This effect is significantly worse at lower incident gamma ray energies, and severely compromises image resolution below several hundred keV.
In accordance with the present invention, high-resolution three-dimensional images of regions-of-interest are provided by use of Compton scattering of gamma rays emitted simultaneously or nearly simultaneously (e.g. within one microsecond) by a radio-nuclide. For exemplary purposes, a general case where three gamma rays are emitted simultaneously is used to describe the production of the images. Compton scattering of the gamma rays is detected by a plurality of position-sensitive detectors that provide a scattering medium and are adapted to detect the location of Compton interactions therein. In addition, some or all of the detectors are adapted to measure energy deposited at the interaction sites. Compton direction cones can be determined for each of the three initial gamma rays as a function of detected Compton scattering interactions and the energy losses of the gamma rays at various gamma-ray interaction locations. The intersection of the three conical surfaces of the Compton direction cones defines a limited number of points, one of which is the location of the radio-nuclide decay. Therefore, a single nuclear decay event which emits three coincident gamma rays provides the location of the emitting radio-nuclide. The accumulation of these detected radio-nuclide decay locations on an event-by-event basis permits the three-dimensional imaging of the region-of-interest. Possible uses of the present invention include providing high-resolution three-dimensional images of radio-pharmaceuticals in medical research and clinical applications and Compton medical imagers that limit the dose of radiation to which a patient is exposed.
According to one aspect of the present invention, an imaging device is provided for generating three-dimensional images from a radio-nuclide emitting first, second and third initial gamma rays as simultaneous or nearly simultaneous emissions resulting from a single nuclear decay event. The imaging device comprises a plurality of position-sensitive gamma ray detectors adapted to determine the locations and energy deposits at interaction sites at which gamma rays undergo Compton scatter and photoelectric interactions. A processor determines a radio-nuclide location as a function of (i) at least two energy values corresponding to each of the first, second and third initial gamma rays, (ii) detected locations of first Compton interaction sites of the first, second and third initial gamma rays, respectively, and (iii) detected locations of second interaction sites of Compton scattered gamma rays corresponding to the first, second, and third initial gamma rays, respectively. The at least two energy values are selected from the group consisting of the energy of a respective initial gamma ray, a first deposition energy of the respective initial gamma ray at the respective first interaction site, the energy of the first Compton scattered gamma ray of the respective initial gamma ray, and a second deposition energy of the corresponding Compton scattered gamma ray at the respective second interaction site. The processor generates a three-dimensional image by superposition of individual radio-nuclide locations.
According to another aspect of the present invention, a system is provided for generating three-dimensional images. The system comprises a radio-nuclide emitting first, second and third initial gamma rays as simultaneous or nearly simultaneous emission resulting from a single nuclear decay event and a plurality of position-sensitive gamma ray detectors. Each detector is adapted to determine the locations of Compton interaction sites at which gamma rays undergo Compton scatter interactions. A processor determines a radio-nuclide location as a function of (i) at least two energy values corresponding to each of the first, second, and third initial gamma rays, (ii) detected locations of first interaction sites of the first, second and third initial gamma rays, respectively, and (iii) detected locations of second interactions sites of Compton scattered gamma rays corresponding to the first, second, and third initial gamma rays, respectively. The at least two energy values are selected from the group consisting of the energy of a respective initial gamma ray, a first deposition energy of the respective initial gamma ray at the respective first interaction site, the energy of the first Compton scattered gamma ray of the respective initial gamma ray, and a second deposition energy of the corresponding Compton scattered gamma ray at the respective second interaction site. The processor generates a three-dimensional image by superposition of individual radio-nuclide locations.
According to another aspect of the present invention, an imaging device is provided for generating three-dimensional images by detecting initial gamma rays emitted as simultaneous or nearly simultaneous emissions resulting from a single nuclear decay event of a radio-nuclide. Subsequent Compton scattered gamma rays result from Compton scattering of initial gamma rays, respectively. The imaging device comprises a first position-sensitive detector array for providing a Compton scattering medium for the initial gamma rays to interact at a respective first interaction site in the first-position sensitive detector array and to thereby generate Compton scattered gamma rays, and for detecting a respective location of each first interaction site. A second position-sensitive detector array, surrounding the first position-sensitive detector array, is for detecting a respective location of each respective second interaction site in the second position-sensitive detector array at which the Compton scattered gamma rays interact. A processor determines a radio-nuclide location as a function of (i) at least two energy values corresponding to each of the first, second and third initial gamma rays, (ii) the respective locations of each detected first interaction sites, and (iii) the location of the second interaction site where the Compton scattered gamma ray corresponding to the first initial gamma ray interacts by photoelectric full energy absorption with the second position-sensitive detector array. The at least two energy values are selected from a group consisting of the energy of the respective initial gamma ray, a first deposition energy of the initial gamma ray at the respective first interaction site, the energy of the first Compton scattered gamma ray of the respective initial gamma ray, and a second deposition energy of the respective Compton scattered gamma ray of the first initial gamma ray at the respective second interaction site. The processor generates a three-dimensional image of the region-of-interest by superposition of individual radio-nuclide locations.
According to another aspect of the present invention, a method is provided for generating three-dimensional images. The method comprises the steps of providing a radio-nuclide source generating three initial gamma rays as simultaneous or nearly simultaneous emissions resulting from a single nuclear decay event and using a first position-sensitive detector array to detect locations of three first interaction sites at which the three initial gamma rays respectively interact with the first position-sensitive detector array. Three Compton scattered gamma rays are generated from the three initial gamma rays, respectively, as a result of the three initial gamma rays interacting with the position-sensitive detector array. A second position-sensitive detector array detects locations of three second interaction sites where the three Compton scattered gamma rays interact by full energy deposition with the second position-sensitive detector array, respectively. A radio-nuclide location is determined as a function of (i) at least two energy values corresponding to each of the three initial gamma rays, (ii) the location of each detected first interaction site, and (iii) the location of each detected second interaction site. The at least two energy values are selected from the group consisting of the respective gamma ray energy of the respective initial gamma ray, a first deposition energy of the initial gamma ray at the respective first interaction site, the energy of the first Compton scattered gamma ray of the respective initial gamma ray, and a second deposition energy of the Compton scattered gamma ray at the respective second interaction site. A three-dimensional image is produced by superposition of individual radio-nuclide locations.
In accordance with yet another aspect of the present invention, a method is provided for generating three-dimensional images comprising the steps of providing a radio-nuclide source generating nuclear decay as a positron and a first coincident gamma ray. The positron annihilates with an electron to thereby generate a second gamma ray and a third gamma ray. A first position sensitive detector array detects locations of first, second, and a third gamma ray first interaction sites where the first, second and third gamma rays interact with the first position-sensitive detector array, respectively. A first Compton-scattered gamma ray is generated from the first gamma ray as a result of the first gamma ray interacting with the first position-sensitive detector array. A second position-sensitive detector array detects the location of a second interaction site where the first Compton scattered gamma ray interacts. The energies of the second and third gamma rays are determined to be consistent with 511 keV from the energy losses at the respective first interaction sites and the respective locations of the first and second interaction sites if a Compton scattered interaction occurred at the first interaction site, or from a full energy loss at the first interaction site if a photoelectric interaction occurred at the respective first interaction site. A radio-nuclide location is determined as an intersection of a Compton direction cone corresponding to the first gamma ray and a line connecting the second gamma ray first interaction site and the third gamma ray first interaction site. The three-dimensional images are produced by superposition of individual radio-nuclide locations.
In accordance with another aspect of the present invention, a method is provided for generating three-dimensional images comprising the steps of providing a radio-nuclide source generating nuclear decay as a first coincident gamma ray and a second coincident gamma ray and administering to the radio-nuclide source a thin region-of-interest. A first position-sensitive detector array is used to detect a location of a first gamma ray first interaction site at which the first gamma ray interacts with the first position-sensitive detector array and the location of a second gamma ray first interaction site at which the second gamma ray interacts with the first position-sensitive detector array. A first Compton scattered gamma ray and a second Compton scattered gamma ray are generated from the first gamma ray and second gamma ray, respectively, as a result of the first gamma ray and second gamma ray interacting with the first position-sensitive detector array, respectively. A second position-sensitive detector array is used to detect a first gamma ray second interaction site at which the first Compton scattered gamma ray interacts with the second position-sensitive detector array and a second gamma ray second interaction site where the second Compton scattered gamma ray interacts with the second position-sensitive detector array. A radio-nuclide location is determined as an intersection of a first Compton direction cone corresponding to the first gamma ray, a second Compton direction cone corresponding to the second gamma ray, and the thin region-of-interest. A three-dimensional image is produced by superposition of individual radio-nuclide locations.
In accordance with yet another aspect of the present invention, a system is provided for generating three-dimensional images. This system comprises a radio-nuclide emitting a first, second and third initial gamma ray as simultaneous or nearly simultaneous emission resulting from a single nuclear decay event. A first position-sensitive detector array provides a Compton scattering medium for the first, second and third initial gamma rays to interact to thereby produce first, second and third Compton scattered gamma rays, respectively. The first position-sensitive detector array detects a respective location of each of the first, second and third initial gamma rays at a first, second and third initial gamma ray first interaction site, respectively, in the first position-sensitive detector array. A second position-sensitive detector array surrounds the first position-sensitive detector array and detects locations of the first, second and third Compton scattered gamma rays at a first, second and third Compton gamma ray second interaction site, respectively, in the second position-sensitive detector array. A processor determines a radio-nuclide location as a function of (i) at least two energy values corresponding to each of the three initial gamma rays, (ii) the location of each detected first interaction site, and (iii) the location of the first Compton gamma ray second interaction site. The at least two energy values are selected from the group consisting of the energy of the respective initial gamma ray, a first deposition energy of the initial gamma ray at the respective first interaction site, the energy of the first Compton scattered gamma ray of the respective initial gamma ray, and a second deposition energy of the first Compton scattered gamma ray at the respective second interaction site. The processor generates a three-dimensional image by superposition of individual radio-nuclide locations.
According to another aspect of the present invention, a method is provided for generating three-dimensional images. The method comprises the steps of providing a radio-nuclide source generating three initial gamma rays as simultaneous or nearly simultaneous emissions resulting from a single nuclear decay event and detecting locations of three first interaction sites at which the three initial gamma rays respectively interact with a plurality of position-sensitive detectors. Three Compton scattered gamma rays are generated from the three initial gamma rays, respectively, as a result of the three initial gamma rays interacting with the position-sensitive detector array. The plurality of position-sensitive detectors detect locations of three second interaction sites where the three Compton scattered gamma rays interact with the plurality of position-sensitive detectors, respectively. A radio-nuclide location is determined as a function of (i) at least two energy values corresponding to each of the three initial gamma rays, (ii) the location of each detected first interaction site, and (iii) the location of each detected second interaction site. The at least two energy values are selected from the group consisting of the respective gamma ray energy of the initial gamma ray, a first deposition energy of the initial gamma ray at the respective first interaction site, the energy of the first Compton scattered gamma ray of the respective initial gamma ray, and a second deposition energy of the Compton scattered gamma ray at the respective second interaction site. A three-dimensional image is produced by superposition of individual radio-nuclide locations.
In accordance with yet another aspect of the present invention, a method is provided for generating three-dimensional images comprising the steps of providing a radio-nuclide source generating nuclear decay as a positron and a first coincident gamma ray. The positron annihilates with an electron to thereby generate a second gamma ray and a third gamma ray. A plurality of position-sensitive detectors detect locations of first, second, and a third gamma ray first interaction sites where the first, second and third gamma rays interact with the first position-sensitive detector array, respectively. A first Compton-scattered gamma ray is generated from the first gamma ray as a result of the first gamma ray interacting with the plurality of position-sensitive detectors. One of said plurality of position-sensitive detectors detects the location of a second interaction site where the first Compton scattered gamma ray interacts. The energies of the second and third gamma rays are determined to be consistent with 511 keV from the energy losses at the respective first interaction sites and the respective locations of the first and second interaction sites if a Compton scatter interaction occurred at the first interaction site, or from a full energy loss at the first interaction site if a photoelectric interaction occurred at the respective first interaction site. A radio-nuclide location is determined as an intersection of a Compton direction cone corresponding to the first gamma ray and a line connecting the second gamma ray first interaction site and the third gamma ray first interaction site. The three-dimensional images are produced by superposition of individual radio-nuclide locations.
An important feature of one form of the present invention concerns the use of a radio-nuclide which has three coincident gamma rays produced from a single nuclear decay event. As a result, the reconstructed position for each decay event is limited to a few small volumes of space, typically one to three with a maximum of eight, but with typically one in the region-of-interest.
One advantage of using the coincident Compton device of the present invention is that a collimator is not required. Therefore, the present invention is considerably more efficient than SPECT which typically requires collimators with 0.001-0.01% transmission efficiencies.
An additional advantage of the present invention is the improved imaging resolution as compared with other Compton imager concepts and techniques.
A further advantage of the present invention is that a patient is subjected to a lower dose of radiation during imaging. The combination of high efficiency and much better definition of a possible source location, i.e. volume of space, as compared with SPECT, results in a lower patient dosage while achieving images of similar or improved quality.
A further feature of the present invention is the reduced Compton scattering in the patient. Compton scattering in the patient can limit the imaging capability of nuclear medicine systems. For the widely-used 99mTc radio-pharmaceuticals, the gamma ray used has an energy of 140 keV. The probability of Compton scattering of the 99mTc gamma rays is about twice that of 94Tc (a candidate for use as a radio-nuclide in the invention) gamma rays per unit path length in body tissue.
Yet a further feature of the present invention is the higher efficiencies achieved using 94Tc thereby resulting in image acquisition times reduced by a factor of 1000 compared by 99mTc SPECT. As a result, images may be acquired in a reduced time while enabling dynamic imaging of various organ functions such as heart function.
Lower Doppler broadening uncertainty and improved image resolution is provided by the present invention when incorporating the use of high-energy gamma ray lines.
Additional features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.